1. Field of the Invention
The invention relates to polarization stabilization, more especially to methods and devices for stabilizing with a high accuracy the polarization state of an optical radiation of arbitrary, possibly time variant, polarization.
2. Description of the Related Art
A polarization stabilizer is a device that transforms an input optical beam having an input state of polarization (SOP) into an output optical beam with a predetermined SOP and an optical power, both not dependent on the input SOP. In general, a defined SOP is determined by two parameters: the ellipticity and the polarization azimuth. Such a device is useful, for example, in coherent optical receivers for matching the SOP between the signal and the local oscillator, in fiber optic interferometric sensors, in compensation of polarization mode dispersion of the transmission line and in optical systems with polarization sensitive components. An important requirement is the endlessness in control, meaning that the stabilizer must compensate in a continuous way for the variations of input SOP.
In polarization division multiplexing (PolDM) transmission at least two optical channels, each comprising an optical carrier, are launched orthogonally polarized in the optical transmission medium, such as for example an optical transmission fiber. In a typical solution for PolDM transmission, the two optical carriers of the at least two orthogonally polarized optical channels are spectrally closely spaced, such as for example within an optical spectrum spacing of 50 GHz or within a 25 GHz spacing. In a preferred configuration, the two carriers, and hence the two channels, have substantially the same optical wavelength. Typically, while the reciprocal orthogonality of the state of polarization is substantially preserved along the propagation into the transmitting medium, the absolute SOPs of the two channels randomly fluctuate at a given position along the line, such as for example at the receiver section.
In PolDM, a problem arises at the receiving section, or whenever the two orthogonally polarized channels need to be polarization demultiplexed. In general, the polarization demultiplexer is typically a polarization beam splitter, which is apt to split two orthogonal SOPs. In case of an error in polarization locking, a misalignment occurs between the SOPs of the two channels and the orthogonal SOPs divided by the polarization demultiplexer. In this case a cross-talk is generated due to an interference between a desired channel and the small portion of the other non-extinguished channel, which severely degrades the quality of the received signal. For example, in PolDM systems having the individual channels intensity modulated with non-return-to-zero format and directly-detected (IM-DD), the penalty to the bit-error-rate becomes about 1 dB for cross-talk of about 20 dB. This means that in case the intensity of the non-extinguished channel is greater than or equal to about 1% of the intensity of the demultiplexed channel, the cross-talk becomes a concern.
Accordingly, in PolDM systems a highly accurate polarization stabilization of the SOPs of the two polarization multiplexed channels is needed before polarization demultiplexing. The cross-talk after polarization demultiplexing is related to the accuracy of polarization stabilization. In case of a single optical channel, the accuracy of a polarization stabilizer in terms of optical power may be expressed through a parameter, called uniformity error, defined according to
                              U          =                                                    I                                  ma                  ⁢                                                                          ⁢                  x                                            -                              I                                  m                  ⁢                                                                          ⁢                  i                  ⁢                                                                          ⁢                  n                                                                                    I                                  ma                  ⁢                                                                          ⁢                  x                                            +                              I                                  m                  ⁢                                                                          ⁢                  i                  ⁢                                                                          ⁢                  n                                                                    ,                            (        1        )            wherein Imax and Imin are the actual maximum and minimum optical intensities, in locked operation, of the polarization-stabilized output radiation of the channel when varying the input SOP. In general, the smaller is the uniformity error, the smaller results the cross-talk after demultiplexing. For example, under simplified conditions, a uniformity error of about 1% gives rise to a cross-talk of about 2%. The patent application US2004/0016874 discloses (see FIG. 4 thereof) an automatic polarization controller for a polarization multiplexed optical pulse train including at least one dither modulation signal, the polarization controller including a polarization transformer of any type. A polarization selective element receives the transformed polarization multiplexed optical pulse train and passes a polarized optical pulse train including the dither modulation signal. A detector receives the polarized optical pulse train including the dither modulation signal and generates a signal that is proportional to the amplitude of the dither modulation signal. A feedback control unit generates a control signal that is coupled to the control input of the polarization transformer.
The patent application US2002/0191265 discloses (see FIG. 3 thereof) a two-stage electro-optic polarization transformer for transforming the polarization states of an orthogonally polarized polarization multiplexed optical signal comprising a first and a second component. An optical feedback signal is extracted from the output of the second stage polarization transformer. In one embodiment, the first and the second components of the polarization multiplexed optical signal are identified with different dither frequencies. A mixer generates a signal that has a frequency that identifies the component of the polarization multiplexed optical signal.
The Applicant has noted that the polarization controllers disclosed in both the documents above directly detect the SOP of the optical radiation only downstream the polarization transformer itself (only downstream the second stage in the second document) and send a single feed-back signal to the feedback control unit. The methods disclosed thus require complicate elaboration of the electrical feedback signal and complicate control algorithm, without adding in precision to the polarization stabilization.
WO03/014811 patent discloses an endless polarization stabilizer based on a two-stage configuration wherein the two stages are controlled independently by an endless polarization stabilizing method based on a simple feedback control algorithm. Each stage comprises a pair of birefringent components that each have fixed eigenaxes and variable phase retardation. The two birefringent components are variable retarders with finite birefringence range and respective eigenaxes oriented at approximately ±45 degrees relative to each other. The endlessness is obtained by commuting the phase retardation of one retarder, when the retardation of the other retarder reaches a range limit.